Four Marines are presumed dead after their CH-53E Super Stallion helicopter crashed in California during a training mission – the second Marine Corps aircraft crash on the same day, officials said.

The helicopter from the 3rd Marine Aircraft Wing went down near El Centro, California, around 2:35 p.m. local time on April 3, the Marines announced Wednesday. The names of the four crew members presumed killed will be released after their next of kin have been notified.

The cause of the crash is under investigation and no further information about what happened was immediately available on Wednesday morning.

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Also on April 3, a Marine Corps AV-8B Harrier crashed during takeoff from Djibouti Ambouli International Airport around 4 p.m. local time, a spokesman for U.S. Naval Forces Central Command said.

The pilot ejected and was listed in stable condition at Camp Lemonnier’s expeditionary medical facility, said Cmdr. Bill Urban. The Harrier came from Marine Medium Tiltrotor Squadron 162, based out of Marine Corps Air Station New River, North Carolina. It was embarked aboard the USS Iwo Jima.

In total, 13 U.S. service members have been killed in separate aircraft crashes since March 14: Two Navy aviators died when their F/A-18F Super Hornet crashed off Florida, and seven airmen were killed on March 15 when their Air Force HH-60 crashed in western Iraq, near the Syrian border.

The combined effects of budget cuts, apathy from Congress, delays in new aircraft programs, and the wear and tear of 17 years of war have made it more dangerous for military aircraft to fly – especially planes and helicopters that continue to fly long after they were supposed to be retired.

The problem became so bad that Defense Secretary James Mattis told the military branches to stop talking about readiness shortfalls in March 2017.